Helium and Radon as Tracers of Groundwater Input Into Florida Bay

Abstract

Dissolved helium and radon anomalies are used to quantify groundwater input to Florida Bay waters. The method relies on the fact that groundwater dissolves large quantities of ⁴He and ²²²Rn isotopes, radioactive decay products of the uranium-chain elements, which accumulate over geological time periods. Seasonal surveys in Florida Bay show average helium concentration anomalies of 13.6% and 16.5% in the summer and winter, respectively. Excess ²²²Rn, in excess of that in equilibrium with ²²⁶Ra, was found to vary from 3 dpm.L^-1 (disintegration per minute per liter) in the summer to 2 dpm.L^-1 in the winter. The fact that such anomalies are present in the 1.5 m deep unstratified waters is a strong indication of groundwater input to the Bay. A simple box model based on helium data yields a groundwater flux of 2.5-4.0 cm.d^-1 In the summer and 10-16 cm.d^-1 in the winter while the same model results in a flux of 0.8-1.7 cm.d^-1 using radon data. The difference between the flux figures obtained from helium and radon may be explained by the two-layer structure of the aquifer system underlying Florida Bay.